The invention relates to the field of substrate processing. More specifically, the invention relates to a method and related apparatus for continuously determining the reaction efficiency of a CVD (chemical vapor deposition) reactor. The reaction efficiency can be used to determine an in-situ growth rate of a metal organic or other material on a substrate, as well as provide quantitative information relating to reacted and unreacted components exhausted from the process reactor.
The process of chemical vapor deposition (CVD) is extensively used to grow layers of various thicknesses of metals, semiconductors, dielectrics and the like. The CVD process typically requires that the desired growth materials be attached to a ligand or volatile adduct that allows the transport of the desired species in the gas phase to a reaction zone in a reactor where a substrate(s) is located. This complex molecule is referred to as the precursor. Different materials have different precursor structures.
Once in the reaction zone, a portion of the volatile precursor compound is decomposed, separating the volatile portion of the compound from the non-volatile portion, and leaving behind the desired solid deposit on the substrate. Typically, the decomposition reaction is driven thermally; that is, the substrate is heated to a sufficiently high temperature such that when the volatile compound contacts the substrate, sufficient energy is made available to break the connectivity between the volatile ligand and the desired atom. The desired atom remains deposited on the substrate, while the volatile portion of the precursor gas is then exhausted from the reactor through an exit port. While thermal energy is the most obvious means for performing the above deposition reaction, it should be noted that CVD explicitly includes processes assisted by other excitation means, including plasma discharge and photo-assisted means, among others, both in the reactor and by upstream excitation of the source gases before they enter the reactor.
The engineering design of CVD process reactors is based on: (1) the transport and even distribution of the volatile species over the entire area of the substrate and (2) the creation of an even temperature profile over the entire surface of the substrate. Should these two criteria be met, the desired material will grow at a predictable rate evenly over the substrate. With experience, the producer learns the proper settings of precursor flow, temperature, and time to produce the correct thickness of a desired film.
To date, however, there is no apparatus available to a producer to verify that each of the learned parameters of flow and temperature are correct. Therefore, the producer must verify that the thickness is correct through trial and error after a growth run is complete and the substrate(s) have been removed from the reactor.
In addition, no means currently exists for determining a process reactor""s utilization efficiency; that is, to determine how much of the precursor gas is consumed. This particular metric is increasingly important for controlling the cost of consumables. Utilization efficiency is also important to the environmentally conscious in order to understand information about the products which are exhausted from the CVD reactor.
A primary object of the present invention is to determine the exhausted reacted and unreacted by-products of any CVD reaction process.
Another primary object of the present invention is to improve the efficiency of chemical vapor deposition (CVD) process reactors.
Another primary object of the present invention is to quantitatively deduce the efficiency of a CVD reaction without relying on empirical data.
It is still another primary object of the present invention to be able to determine an in-situ growth rate and thickness of the thin film onto a substrate in a CVD reactor.
An example is described for oxygen and TEOS yielding silicon dioxide in a plasma enhanced CVD reactor (PECVD) per the following:
Si(OC2H5)4+5O2xe2x86x92SiO2+2CO+10H2O
in which SiO2 is the solid product. The reaction inputs will generally have excess oxygen; that is, greater than stoichiometric, to facilitate the growth of high quality material in a plasma process. Therefore, the exhaust stream will have:             w      5        ⁢    CO    +      w    ⁢          xe2x80x83        ⁢          H      2        ⁢    0    +            x      1        ⁢                  Si        ⁡                  (                                    OC              2                        ⁢                          H              5                                )                    4        +      y0    2    +            z0      3        .  
in which w, x1, y, and z are related to the mass quantities of gases in the exhaust stream. The quantities of CO and H2O are related in the ratio of 1:5 because they predominantly result from the disassociation reaction, the abundance of these species resulting from wall desorption or outgassing being insignificant.
The present invention can also be applied to thermal CVD processes conventionally carried out in hotwall quartz furnace tubes including, but not limited to, the following conventionally standard manufacturing processes:
oxide from TEOS at low pressure; PSG, BSG, BPSG, i.e.: TEOS oxide with boron and/or phosphorous doping; silane-based oxide (using SiH4 or higher silanes, with O2 or N2O); silane-based nitride (using SiH4 or higher silanes, with NH3); polysilicon or silicon epitaxy from SiH4, Si22H6, or other silanes, or from Cl-containing silicon precursors (e.g.: SiCl4, SiClH3, etc.).
According to yet another preferred aspect of the invention, there is provided an apparatus for determining the reaction efficiency of a CVD reactor, said reactor having a defined cavity for retaining at least one substrate onto which is deposited a solid reaction product from a reacted binary gas mixture (aA+bB⇄cC+dD), said reactor further including an inlet port and an outlet port, said apparatus comprising:
means for determining the composition of said gas mixture at the outlet port of the reactor;
means for determining the composition of said gas mixture at the inlet port of the reactor;
wherein said outlet composition determining means is an acoustic cell capable of measuring relative differences in the speed of sound for gases passing therethrough, in which the reacted product is derived from the relation:             (                                    c            _                    o                          c          _                    )        2    =                                          (                          1              -              Δ                        )                    ⁢                      M            A                          +                              (                          x              -                                                b                  a                                ⁢                Δ                                      )                    ⁢                      M            B                          +                  c          ⁢                      Δ            a                    ⁢                      M            C                                                M          A                +                  x          ⁢                      xe2x80x83                    ⁢                      M            B                                ·                            [                                    1              +              x                                      1              +              x              +                                                Δ                  a                                ⁢                                  (                                      c                    -                    b                    -                    a                                    )                                                              ]                2            ⁡              [                                                            1                -                Δ                                            γ                A                                      +                                          x                -                                                      b                    a                                    ⁢                  Δ                                                            γ                B                                      +                          c              ⁢                              Δ                                  γ                  c                                                                          (                                          1                /                                  γ                  A                                            +                              x                /                                  γ                  B                                                      )                          ]            
in which:
xcex94=reacted product(reacted fraction of species A);
MA, MA, MC=molecular weights of species A, B, C;
YA, YB, YC=specific heat ratios of A, B, C;
x=mole fraction of A in B; and   (                    c        _            o              c      _        )
xe2x80x83is the frequency ratio of the gas mixture between a reacted mode and a bypass mode.
According to a preferred embodiment, a pair of acoustic cells are used to determine gas compositions at the inlet and outlet sides of the reactor, respectively.
As is known, an acoustic cell can be used to determine the composition of a binary gas mixture from knowledge of the molecular weights of the gases in a particular gas mixture, their individual specific heat ratios (Cp/Cv) and the measurement of the speed of sound. A more complete description of an acoustic cell used in conjunction with chemical vapor deposition processes can be found in commonly owned U.S. Pat. No. 5,768,937, the entire contents of which are hereby incorporated by reference.
According to this invention, and by calculating the consumption of precursor gas within the reactor based on a specific chemistry; (e.g., the reaction efficiency), a number of benefits can be derived. First, the growth rate of thin film material can be determined from quantitative information derived from the pair of acoustic cells, disposed at the inlet and the outlet of the reactor, and a knowledge of the reaction chemistry or some other independent empirical determination of the absolute efficiency of the acoustic cell measurement.
Second, by understanding the reaction efficiency, and by use of the acoustic cells which are capable of determining gas composition data at the outlet side of the reactor, information relating to the exhausted gas composition can be derived. Knowledge of the species and quantity of emission gases allows determination of this xe2x80x9cefficiencyxe2x80x9d parameter, which is becoming increasingly important from an environmental standpoint stemming from growing awareness of producers, as well as promulgated standards implemented by groups such as the Environmental Protection Agency, among others.
According to a preferred embodiment of the invention, a frequency ratio related to the speed of sound (c/co) is measured at the outlet side of the reactor by a first acoustic cell while a second acoustic cell measures the composition of the binary gas combination at the inlet; that is, an arriving composition ratio at the reactor inlet. According to the present method, a derivation of the reaction fraction (xcex94) can then be calculated from this data alone for each reaction process. The above information provides the reactor""s efficiency. Using the derived information for the quantity of the reacted product, along with independent knowledge of the inlet""s total molar flow rate, the total area of the substrate, and other known information related to the structure of the resulting thin film, the growth rate of the thin film can then also be derived.
In addition to deriving the thin film""s growth rate, information pertaining to the reacted and unreacted products exhausted from the reactor can also be derived using the reaction efficiency, as well as known information pertaining to the mass flow entering the reactor chamber and the dominant chemical reaction in a manner described in greater detail below.
Alternately, the above information could also be obtained using a single acoustic cell at the outlet side of the reactor and by either reducing the temperature or otherwise turning off the reaction, or by adding a by-pass line.
An advantage of the present invention is that the thin film growth rate onto a substrate resulting from a chemical vapor deposition (CVD) reaction can be calculated easily and quickly using known data and at least one acoustic cell capable of determining the composition of the gas mixture.
Another advantage of the present invention is that a quantitative analysis of the reaction at the inlet and outlet sides of the reactor is useful in calculating the reaction efficiency and thereby determining information relating to the exhaust product, including the presence of unreacted gases resulting from an uncompleted reaction.
These and other objects, features, and advantages will now be described in the following Detailed Description which should be read in conjunction with the accompanying drawings.